1. Field of the Invention
The present invention relates to a shadow mask for a color cathode ray tube used in, for example, a television or a computer display. It relates in particular to a slotted shadow mask in which a plurality of slot apertures are formed in a flat panel as passing apertures for an electron beam.
2. Description of the Prior Art
FIG. 5 shows a cross-sectional view illustrating an example of a conventional color cathode ray tube. The color cathode ray tube 1 shown in this drawing includes a substantially rectangular face panel 2 on whose inner surface a phosphor screen 2a is formed, a funnel 3 connected to the rear side of the face panel 2, an electron gun 4 provided within a neck portion 3a of the funnel 3, a shadow mask 6 facing the phosphor screen 2a within the face panel 2, and a mask frame 7 fixing the shadow mask 6. In order to deflect and scan an electron beam, a deflection yoke 5 is provided on the peripheral surface of the funnel 3.
The function of the shadow mask 6 is to select colors so that the three electron beams emitted from the electron gun 4, each corresponding to one color, reach the luminescent material of the corresponding color on the phosphor screen 2a. xe2x80x9cAxe2x80x9d illustrates a trajectory of the electron beam. The following is an explanation of a slotted shadow mask as a conventional example of the shadow mask 6. In the slotted shadow mask, a plurality of substantially rectangular slot apertures are formed in a flat panel, by etching, as passing apertures for the electron beams.
FIG. 6 shows a part of a top view illustrating an example of the slotted shadow mask. In this figure, the arrow direction x indicates the horizontal screen direction, and the arrow direction y indicates the vertical screen direction. The slot apertures 8 are formed with constant vertical pitch. The portion 9 between the slot apertures 8 is called a xe2x80x9cbridgexe2x80x9d.
The bridge width affects the mechanical strength of the shadow mask. When the bridge is made narrower, the shadow mask becomes vulnerable to horizontal tension in particular. When the bridge is made wider to improve the mechanical strength, the brightness decreases due to the smaller area of the slot apertures.
Publication of Japanese Unexamined Patent Application (Tokkai) No. Hei 10-40826 suggests that, to achieve both brightness and mechanical strength of shadow mask, a certain numerical relationship is defined between the length of the portion of the bridge that has not been etched and that is common to the front and the back surface (that is, the region where neither the front nor the back of the shadow mask has been etched, and where the shadow mask retains its original thickness), and the vertical pitch of the slot apertures. Also, Publication of Japanese Examined Patent Application (Tokko) No. Hei 4-74818 discloses an improvement of the mechanical strength by making the bridges at a peripheral portion of the shadow mask wider than those of a central portion because of the stress concentrations in the peripheral portions of the shadow mask in the molded mask made by press forming.
However, shadow masks of conventional color cathode ray tubes as explained above lead to the following problems:
(1) Considering the mechanical strength of the shadow mask, the bridge requires portions that have not been etched and that are common to the front and the back surface. Therefore there is a certain limit to how narrow the bridge can be made and how much the brightness can be improved. In addition, when the bridges in the peripheral portion of the shadow mask are made wider than those in the central part, the mechanical strength improves, but the brightness at the peripheral portion decreases.
(2) The phosphors are arranged in stripes. Therefore, when the shape of slot apertures is round, the shape of the electron beam also becomes round when it has passed the slot aperture. This is not preferable with regard to improving the brightness. In addition, shaking of the shadow mask due to shock causes misalignment of the electron beam. In this case, there is a significant change in brightness and color purity.
In order to solve these problems, it is an object of the present invention to provide a color cathode ray tube with a one-dimensional tension type shadow mask, which is stretched applying a tension to one direction, and whose bridges are made narrower to a certain degree, thus improving the brightness without compromising the strength of the shadow mask.
In order to achieve the object mentioned above, a color cathode ray tube according to the present invention has a color selection electrode, the color selection electrode including a pair of supports facing each other; a shadow mask provided with a plurality of slot apertures, the shadow mask being stretched and fixed by the pair of supports; and elastic members arranged between the supports, and holding the support. Bridges between vertically adjacent slot apertures are formed on the shadow mask. Defining xe2x80x9cbridge widthxe2x80x9d as the largest vertical width of a bridge, the narrowest bridge width is in a range of about 3-10% of a vertical pitch of the slot apertures. With such a color cathode ray tube, the necessary strength can be ensured, and the brightness can be improved without compromising strength, because the bridge widths can be made narrower.
It is preferable that bridges having bridge width in the range of 3-10% of the vertical pitch of the slot aperture include bridges having maximum thickness less than that of the shadow mask. With such a color cathode ray tube, there are no bridge portions that have not been etched and that are common to the front and the back surface, so that the bridge width can be made as narrow as possible.
It is preferable that the maximum thickness of the bridge having maximum thickness less than that of the shadow mask is at least 50% and less than 100% of the thickness of the shadow mask.
It is preferable that the slot apertures are substantially rectangular. With such a shape, the shape of the electron beam after it has passed the slot apertures is substantially rectangular as well, so that the necessary brightness can be ensured, and the image deterioration due to shaking of the shadow mask can be reduced.
It is preferable that the relationship a/b  greater than 1.8 is satisfied, wherein xe2x80x9caxe2x80x9d is half the horizontal width of one of the slot apertures, and xe2x80x9cbxe2x80x9d is the height of a curved portion formed on upper and lower edges of one of the slot apertures in a vertical direction.
It is preferable that the bridge widths in a central portion of the shadow mask are substantially the same as the bridge widths in a peripheral portion of the shadow mask, or the bridge widths in the peripheral portion of the shadow mask are narrower than the bridge widths in the central portion of the shadow mask. With such an arrangement, a decrease of the brightness in the peripheral portion can be prevented.